This invention generally concerns an improved heat transfer device for use in a circuit board and more specifically is directed to improved transistor amplifier stabilization by the thermal coupling of the amplifier to transistor biasing means.
Semiconductor devices such as transistors are generally subject to operating instabilities due to temperature variations. Semiconductor temperature variations arise from either changes in the ambient temperature of the operating environment or due to power dissipation variations in the semiconductor device itself. Moderate increases in operating temperature of a semiconductor may result in variations in the output of the device. Extreme variations in operating temperature may result in the destruction of the semiconductor device if it is driven into a saturated conducting mode. This is due to the positive temperature conducting coefficient possessed by most semiconductors which produces an increase in device conductivity with increasing temperature. For example, the collector current of a transistor will usually increase with increased operating temperature. Thus, a transistor operating at high ambient temperatures and high heat dissipation will draw more collector current as the temperature of the collector junction increases. This rise in collector current causes a corresponding increase in collector junction temperature, and, under certain conditions, the process becomes cumulative and continues until the transistor is destroyed. For example, unless heat dissipation is provided for, it is possible that a silicon transistor which is operating at low heat dissipation at a - 55.degree. C. will thermally run away when the ambient temperature is increased to 30.degree. C.
The prior art discloses various approaches to controlling the thermal conductivity of a semiconductor device to avoid device destruction caused by thermal runaway. These approaches include DC feedback for biasing the transistor toward cutoff as the collector current increases, effective means for cooling the collector junction, and the temperature compensation of transistor bias by means of thermistors. The first approach generally involves a negative current feedback, similar to cathode bias in electron tubes, in which a resistor connected between emitter and ground provides a reverse bias which, as the collector current tends to increase with increasing temperature, changes the base current in such a way as to stabilize the collector current. This technique is especially useful for temperature compensation in certain types of transistor amplifiers such as audio frequency balanced or push-pull power amplifiers for class B operation wherein the transistors are to be maintained essentially at cutoff in a zero signal condition. This transistor biasing approach, while effective, generally requires the addition of a somewhat complex feedback control circuit to the existing semiconductor circuit layout thus increasing system complexity and cost. In addition, the semiconductor components in the negative feedback circuit must possess approximately the same temperature conducting characteristics as possessed by those components in the semiconductor circuit to which the bias correction is to be applied and the thermal environments of the temperature-sensitive components in both circuits must be as nearly identical as possible for proper control signal feedback.
One example of the use of a DC feedback signal for biasing a transistor toward cutoff as the collector current rises is disclosed in U.S. Pat. No. 2,951,208. This temperature controlled semiconductor bias circuit applies a voltage drop across a diode to the bases of a pair of coupled transistors in a standard transistor amplifier. The voltage applied to the transistor bases is negative relative to the voltage of the respective emitter electrodes of the transistors. An increase in the ambient temperature results in an increase in diode temperature and a decrease in diode resistance. This decrease in diode resistance will, in turn, produce a decrease in the voltage drop across the diode. In this manner, the forward bias voltage which is applied between the emitters of the coupled transistors and the respective base electrodes is decreased to decrease the collector current and maintain this current at a substantially constant, optimum value despite changes in ambient temperature. Should the temperature decrease, the reverse effect will prevail resulting in a stabilization in collector current. One embodiment of this thermal compensation circuit recites the thermal coupling of the temperature controlled, or responsive, impedance circuit element (diode) with at least one of the controlled transistors. While it is noted that this thermal coupling arrangement would compensate not only for large power dissipation but also for ambient temperature variations, no effective coupling means, or details thereof, is disclosed in the patent. Thus, this approach fails to correct for temperature differentials between the controlled semiconductor and the current biasing feedback device.
Another technique is to compensate for the temperature dependency of a transistor circuit element with another temperature-dependent circuit element such as a thermistor. In the case of a power transistor, it is desirable to have the thermistor coupled to the collector junction through a path with negligible thermal drop thus ensuring that the two temperature complementing semiconductor devices are operating at the same temperature. This approach, however, suffers from the limitations of the earlier-discussed invention in that no high efficiency thermal transfer device or system between temperature compensating semiconductor devices as currently available.
Still another approach to solving the problem of the dependence of semiconductor conductivity upon dissipation and ambient temperatures involves the use of cooling means to reduce the collector junction temperature. Thermal dissipation may be accomplished by refrigeration, forced-air cooling, and, more commonly, by radiation, conduction and convection to the surroundings. For a given temperature rise the amount of heat that can be dissipated is determined by the size of the device and, to a lesser extent, by its shape. In general, the semiconductor is thermally joined to a metal chassis or some other heat sink to increase thermal dissipation. Power transistors are typically clamped to the chassis with studs or screws to ensure a low thermal drop between the transistor and the chassis. A power transistor clamped to a heat sink may have a thermal rise above ambient temperature as low as 2.degree. C. per watt.
One approach for improving the transfer of heat from a heat source to a heat sink is disclosed in U.S. Pat. No. 4,151,547. This approach involves the incorporation of a malleable, dimpled wafer between a semiconductor device such as a power transistor and an adjacent heat sink. By deforming the wafer so as to exactly overlap the surface of the heat sink in both size and shape, heat transfer from an operating semiconductor device to the heat sink is substantially enhanced. While this approach provides efficient thermal dissipation, it offers no active control of semiconductor operation and the increased accuracy associated therewith, as in the previously discussed bias current feedback systems.
Another approach to the stabilization of temperature-dependent transistor operation is presented in Wakefield SemiConductor Heat Sinks and Thermal Products, Distributor Products Catalog No. 102 (1970 Edition), page 17. Disclosed therein is a thermal equalizing link consisting of two clips bonded together and mounted to coupled transistors which form an output device such as an operational amplifier. This clip maintains the balanced transistors of the amplifier at the same case temperature providing for a more stable temperature-dependent amplifier output. This approach, however, is limited to thermal equalization of amplifier components and fails to address amplifier output control by a device or system which compensates for temperature variations of the amplifier as a unit.
The present invention, however, combines the semiconductor feedback bias current approach with efficient thermal coupling between the controlled semiconductor and the control feedback device in providing for stable semiconductor operation and preventing thermal runaway.